Hologram is one wherein an interference pattern of a coherent light of laser is recorded on a photosensitive material or the like. Since hologram has multiple functions, it is diversely utilized for optical elements, three-dimensional image displays, interference measurement, image/information processing, and the like. Moreover, since hologram can be regarded as an equivalent to information recording at a sub-micron unit, it is widely utilized as marks for preventing counterfeit of securities, credit cards, and the like.
In particular, in a volume phase-type hologram, a phase can be modulated by not optical absorption but formation of spatial interference fringes having different diffractive indices in a hologram recording medium without absorbing a light beam passing through the image, so that it is expected to apply it to hologram optical elements and hologram recording materials in addition to display uses in recent years.
As such hologram recording materials, there have been proposed an photo-curable resin composition wherein constituting components of a photopolymerization initiator, a 3-ketocoumarin and a diaryliodonium salt are used in combination (see, Patent Document 1) and a hologram recording material wherein a photopolymerization initiator and polymethyl methacrylate as a supporting polymer are combined (see, Patent Document 2).
Moreover, as a photopolymerizable sensitive material capable of preparing hologram by one treating step without involving any wet treatment, there have been proposed two types of photosensitive materials, i.e., the following first and second examples (see, Patent Document 3).
The first example is a photosensitive resin composition comprising a combination of two unsaturated ethylenic monomers different in reactivity and refractive index and a photopolymerization initiator, e.g., cyclohexyl methacrylate, N-vinylcarbazole, and benzoin methyl ether and capable of hologram recording by holding them between two glass plates and exposing it by means of a two-beam optical system.
The second example is a photosensitive resin composition comprising four components, i.e., a polymerizable unsaturated ethylenic monomer and an unsaturated ethylenic monomer acting as a crosslinking agent at the polymerization thereof which have similar refractive indices, a non-reactive compound having an refractive index different from those of the above two monomers, and a photopolymerization initiator, e.g., butyl methacrylate, ethylene glycol dimethacrylate, 1-phenylnaphthalene, and benzoin methyl ether and capable of manufacturing hologram as in the case of the first example.
Furthermore, there is a proposal wherein a basic composition is composed of a thermoplastic resin, a polymerizable unsaturated ethylenic monomer, and a photopolymerization initiator and, in order to enhance modulation of refractive index, a difference in refractive index is achieved by the use of a compound having an aromatic ring as either of the thermoplastic resin or the polymerizable unsaturated ethylenic monomer (see, Patent Document 4).
In addition, it is proposed to blend a cation-polymerizable monomer and a cationic polymerization initiator instead of the plasticizer in the above proposal (see, Patent Document 5). Also, there has been proposed a photosensitive resin composition for hologram recording comprising an epoxy resin, a radical-polymerizable unsaturated ethylenic monomer, and a photo radical polymerization initiator (see, Patent Document 6).    Patent Document 1: JP-A-60-88005    Patent Document 2: JP-A-4-31590    Patent Document 3: U.S. Pat. No. 3,993,485    Patent Document 4: U.S. Pat. No. 5,098,803    Patent Document 5: JP-A-5-107999    Patent Document 6: JP-A-5-94014